2. Presented by Simon Chan (1) Celine Ho (9) Celine Ho (9) Agnes Ng (21) Shung Tak Catholic English College Agnes Ng (21) Shung Tak Catholic English College May, 1999

3. The study of energy change during a chemical reaction is known as energetics. What is Energetics? The total energy stored in a substance is called the ‘heat content’ or enthalpy, with symbol H. △ △ H = Enthalpy of products – Enthalpy of reactants △ △ H = H p – H r

4. Heat may be absorbed or evolved. Reactions are therefore divided into two types: endothermic and exothermic. absorbed heat n It is a reaction that absorbed heat from the surroundings and the enthalpy change of the reaction is positive. Endothermic Reaction

5. Exothermic Reaction It is a reaction that releases heat to the surroundings and enthalpy change of the reaction is negative. Exothermic Reaction

6. Applications: Combustion Combustion e.g. alkane burns in oxygen with a non-smoky flame and gives out large amount of heat energy. Combustion of methane:

7. Some lunch boxes make use of the reaction between iron powder and oxygen to heat up the content. Some lunch boxes make use of the reaction between iron powder and oxygen to heat up the content. 4Fe(s)+3O 2 (g)  2Fe 2 O 3 (s)  H=-285.9 kJ mol -1 Instant Heat Lunch BoxInstant Heat Lunch Box Hot Packs It is widely used to release muscular pain.

8. A hot pack is a heat storage device. A hot pack, containing a safe food grade salt., can heat instantly to a safe temperature

9. Why a Hot Pack Is Used?? The advantages of applying hot packs on painful areas to relieve muscular pains have been recommended for years all over the world. It helps in the healing process.

10. Medical Uses of a Hot Pack   In most situations, a hot pack may be stored at the patients bed side to keep them warm. It is simple and safe enough to use, even by the patients themselves.

11. The Principle of the Hot Packs

12. Principle: If a saturated solution remains undisturbed as it is cooled, it often passes the point at which it would normally crystallize, reaching a supersaturated state. It is an equilibrium that the solution turns to crystals and at the same time The crystals turn back to solution.

13. Chain Reaction If there is a change in physical condition, e.g. pressure, the equilibrium position will shift and chain reaction will occur. If there is a change in physical condition, e.g. pressure, the equilibrium position will shift and chain reaction will occur.

14. In fact, there are two types of hot packs --- non-reusable hot pack and reusable hot pack Reusable hot pack---by the crystallization of supersaturated chemical. Reusable hot pack---by the crystallization of supersaturated chemical. Non-reusable hot pack --- adding chemicals together and releasing heat

15. The reusable hot pack is more preferable as: it can be reused until the container breaks; The cost-efficiency ratio is much higher; Also, it is more environmental friendly. Our Choice

16. Preparation of the Hot Pack Using Supersaturated Sodium Thiosulphate Pentahydrate Solution Preparation of the Hot Pack Using Supersaturated Sodium Thiosulphate Pentahydrate Solution ( Na 2 S 2 O 3 · 5H 2 O )? Thiosulphate ion ( S 2 O 3 2- )

17. i). 1-2 grams of sodium thiosulphate pentahydrate is put into a small test tube. The test tube is put into a hot water bath (near boiling).

18. ii). The contents are swirled periodically until all the crystals are converted into liquid. iii). The test tube is allowed to cool to room temperature.

19. iv) The test tube is connected to a syringe. Pump in air and disturbs the equilibrium of the system to initiate chain reaction and heat is released.

20. v).To reuse the chemicals, we can simply heat the crystals until the system completely becomes liquid. Then, allow them to cool and the pack is ready to be reused.

21. Enthalpy Change  H= -50 kJ mol -1

22. The crystal formed in the above reaction may tend to expand and break the test tube. To avoid this problem, we can reheat the test tube to melt the contents to liquid. The hot solution can be quickly cooled in an ice bath if one is in a hurry. The hot pack can be reused until the bag breaks.

23. Our Design: Air in Air bag Tube Container Sodium Thiosulphate Mechanism

24. Data Extracted From: Following Web sites n Chemical Sciences. Solubility Product Constants n Passmores Comprehensive School n UCSD, Chem-Biochem, Undergraduate Teaching Labs n Science Lab n REDOX CHEMICALS PTY LTD. n Virginia Tech Chemistry Department n JCE Software n Microsoft Encarta n Thermo-Pad